Transmission Media / Channels

Transmission Media / Channels

Introduction

• Provides the connection between the transmitter and receiver.1. Pair of wires – carry electric signal.2. Optical fiber – carries the information

on a modulated light beam.3. Free space – information-bearing signal

is radiated by antenna

Cont’d...

• Signal transmission problem additive noise – generated internally by

components used to implement the communication system.

Interference from other users of the channel.

Cont’d...

• Minimizing noise effects – Increasing the power of transmitted

signal.• Constraint

– Limited power level – Channel bandwidth availability

Cont’d...

Cont’d...• Wire Lines Channel

– Signals transmitted are distorted in both amplitude and phase. – corrupted by noise.

– Carry a large percentage of daily communication around the world.

http://en.wikipedia.org/wiki/Coaxial_cablehttp://en.wikipedia.org/wiki/Twisted_pair

Cont’d...

Twisted pair Coaxial cable

Cont’d...• Fiber Optics Channel

– Low signal attenuation– Highly reliable photonic devices– Large bandwidth available– Services : voice, data facsimile and video– Tx – light source

(e.g. LED, laser)– Rx – photodiode– Noise source : photodiodes & amplifiers

http://en.wikipedia.org/wiki/Fiber_optics

Cont’d...

Cont’d...

• Wireless Electromagnetic Channels– Electromagnetic energy is coupled

to the propagation medium by antenna (radiator)

– Antenna size & configuration – Frequency of operation

– Efficient radiation – antenna longer than 1/10 λ

Cont’d...

• Example A radio station transmitting in AM

frequency band, fc = 1MHz, λ = 300 m,

requires antenna at least 30 m.

Cont’d...

Various frequency bands of the electromagnetic spectrum

Cont’d...

• Mode of propagation of EM waves

i. Ground-wave propagationii. Sky-wave propagationiii. Line-of-sight (LOS)

GROUND-WAVE PROPAGATION

• Surface-wave propagation• Dominant mode of propagation• Frequency band: 0.3 – 3 MHz• Applications: AM broadcasting,

maritime radio broadcasting• Disturbances for signal

transmission: atmospheric noise, man-made

noise, thermal noise.

Cont’d...

SKY-WAVE PROPAGATION

• Transmitted signals being reflected from ionosphere

• Frequency : above 30 MHz• Little loss• Problem : Signal Multipath• Application : Satellite

communications

Cont’d...

Antenna at different angles

> fc

LINE-OF-SIGHT (LOS) PROPAGATION

• VHF band and higher• Limited by curvature of earth• Problem : Thermal noise (Rx front end)

Cosmic noise (pick-up by antenna)

• Application: A TV antenna mounted on a tower of 300 m height to provide a broad coverage area (67km)

Cont’d...

Cont’d...• Underwater acoustic channels

– EM waves do not propagate over long distances under water except at extremely low frequencies

– Expensive – because of the large and powerful transmitters required

– Problem : Attenuation – skin depth

Cont’d...• Multipath channel – signals

reflections from the surface and the bottom of the sea.

• Noise : ambient ocean acoustic noise,

man-made acoustic noise

END OF PART 2END OF PART 2